Preheat oven to 350°F and line baking sheets with parchment paper or silicone mats.
Beat butter, granulated sugar, and brown sugar together in a large bowl until creamy and fluffy, about 2-3 minutes.
Add eggs and vanilla, beating well after each addition until fully incorporated.
In a separate bowl, whisk together flour, baking soda, and salt. Gradually add to the wet ingredients, mixing just until combined.
Gently fold in 1 cup of M&Ms and all the chocolate chips, reserving the remaining 1/2 cup M&Ms for topping.
Drop rounded tablespoons of dough 2 inches apart on prepared baking sheets. Press 2-3 extra M&Ms on top of each cookie for that picture-perfect rainbow look.
Bake for 9-11 minutes until edges are lightly golden but centers still look soft. Don't overbake—they'll continue to cook on the pan!
Let cool on baking sheet for 5 minutes before transferring to a wire rack to cool completely.
